Description

COMMENT stores a comment about a
database object.

Only one comment string is stored for each object, so to
modify a comment, issue a new COMMENT
command for the same object. To remove a comment, write
NULL in place of the text string.
Comments are automatically dropped when their object is
dropped.

For most kinds of object, only the object's owner can set the
comment. Roles don't have owners, so the rule for COMMENT ON ROLE is that you must be superuser to
comment on a superuser role, or have the CREATEROLE privilege to comment on non-superuser
roles. Of course, a superuser can comment on anything.

Comments can be viewed using psql's \d family of
commands. Other user interfaces to retrieve comments can be built
atop the same built-in functions that psql uses, namely obj_description, col_description, and shobj_description (see Table
9-56).

Parameters

The name of the object to be commented. Names of tables,
aggregates, collations, conversions, domains, foreign
tables, functions, indexes, operators, operator classes,
operator families, sequences, text search objects, types,
and views can be schema-qualified. When commenting on a
column, relation_name must
refer to a table, view, composite type, or foreign
table.

agg_type

An input data type on which the aggregate function
operates. To reference a zero-argument aggregate function,
write * in place of the list of
input data types.

source_type

The name of the source data type of the cast.

target_type

The name of the target data type of the cast.

argmode

The mode of a function argument: IN, OUT, INOUT, or VARIADIC.
If omitted, the default is IN.
Note that COMMENT ON FUNCTION does
not actually pay any attention to OUT arguments, since only the input
arguments are needed to determine the function's identity.
So it is sufficient to list the IN, INOUT, and
VARIADIC arguments.

argname

The name of a function argument. Note that COMMENT ON FUNCTION does not actually pay
any attention to argument names, since only the argument
data types are needed to determine the function's
identity.

argtype

The data type(s) of the function's arguments (optionally
schema-qualified), if any.

large_object_oid

The OID of the large object.

left_typeright_type

The data type(s) of the operator's arguments (optionally
schema-qualified). Write NONE for
the missing argument of a prefix or postfix operator.

PROCEDURAL

This is a noise word.

text

The new comment, written as a string literal; or
NULL to drop the comment.

Notes

There is presently no security mechanism for viewing comments:
any user connected to a database can see all the comments for
objects in that database. For shared objects such as databases,
roles, and tablespaces, comments are stored globally so any user
connected to any database in the cluster can see all the comments
for shared objects. Therefore, don't put security-critical
information in comments.

Examples

Attach a comment to the table mytable:

COMMENT ON TABLE mytable IS 'This is my table.';

Remove it again:

COMMENT ON TABLE mytable IS NULL;

Some more examples:

COMMENT ON AGGREGATE my_aggregate (double precision) IS 'Computes sample variance';
COMMENT ON CAST (text AS int4) IS 'Allow casts from text to int4';
COMMENT ON COLLATION "fr_CA" IS 'Canadian French';
COMMENT ON COLUMN my_table.my_column IS 'Employee ID number';
COMMENT ON CONVERSION my_conv IS 'Conversion to UTF8';
COMMENT ON CONSTRAINT bar_col_cons ON bar IS 'Constrains column col';
COMMENT ON DATABASE my_database IS 'Development Database';
COMMENT ON DOMAIN my_domain IS 'Email Address Domain';
COMMENT ON EXTENSION hstore IS 'implements the hstore data type';
COMMENT ON FOREIGN DATA WRAPPER mywrapper IS 'my foreign data wrapper';
COMMENT ON FOREIGN TABLE my_foreign_table IS 'Employee Information in other database';
COMMENT ON FUNCTION my_function (timestamp) IS 'Returns Roman Numeral';
COMMENT ON INDEX my_index IS 'Enforces uniqueness on employee ID';
COMMENT ON LANGUAGE plpython IS 'Python support for stored procedures';
COMMENT ON LARGE OBJECT 346344 IS 'Planning document';
COMMENT ON MATERIALIZED VIEW my_matview IS 'Summary of order history';
COMMENT ON OPERATOR ^ (text, text) IS 'Performs intersection of two texts';
COMMENT ON OPERATOR - (NONE, integer) IS 'Unary minus';
COMMENT ON OPERATOR CLASS int4ops USING btree IS '4 byte integer operators for btrees';
COMMENT ON OPERATOR FAMILY integer_ops USING btree IS 'all integer operators for btrees';
COMMENT ON ROLE my_role IS 'Administration group for finance tables';
COMMENT ON RULE my_rule ON my_table IS 'Logs updates of employee records';
COMMENT ON SCHEMA my_schema IS 'Departmental data';
COMMENT ON SEQUENCE my_sequence IS 'Used to generate primary keys';
COMMENT ON SERVER myserver IS 'my foreign server';
COMMENT ON TABLE my_schema.my_table IS 'Employee Information';
COMMENT ON TABLESPACE my_tablespace IS 'Tablespace for indexes';
COMMENT ON TEXT SEARCH CONFIGURATION my_config IS 'Special word filtering';
COMMENT ON TEXT SEARCH DICTIONARY swedish IS 'Snowball stemmer for Swedish language';
COMMENT ON TEXT SEARCH PARSER my_parser IS 'Splits text into words';
COMMENT ON TEXT SEARCH TEMPLATE snowball IS 'Snowball stemmer';
COMMENT ON TRIGGER my_trigger ON my_table IS 'Used for RI';
COMMENT ON TYPE complex IS 'Complex number data type';
COMMENT ON VIEW my_view IS 'View of departmental costs';